The tendency of optical fibers to leak optical energy when bent has been known since the infancy of the technology. It is well known that light follows a straight path but can be guided to some extent by providing a path, even a curved path, of high refractive index material surrounded by material of lower refractive index. However, in practice that principle is limited, and optical fibers often have bends with a curvature that exceeds the ability of the light guide to contain the light.
Controlling transmission characteristics when bent is an issue in nearly every practical optical fiber design. The initial approach, and still a common approach, is to prevent or minimize physical bends in the optical fiber. While this can be largely achieved in long hauls by designing a robust cable, or in shorter hauls by installing the optical fibers in microducts, in all cases the optical fiber must be terminated at each end. Thus even under the most favorable conditions, bending, often severe bending, is encountered at the optical fiber terminals.
Controlling bend loss can also be addressed by the physical design of the optical fiber itself. Some optical fibers are inherently more immune to bend loss than others. This was recognized early, and most optical fibers are now specifically designed for low loss. The design features that are typically effective for microbend loss control involve the properties of the optical fiber cladding, usually the outer cladding. Thus ring features or trench features, or combinations thereof, are commonly found at the outside of the optical fiber refractive index profiles to control bend losses. See for example, U.S. Pat. Nos. 4,691,990 and 4,852,968, both incorporated herein by reference.
Performance issues for optical fibers under bend conditions have generally been considered to involve generalized optical power loss, due to leakage of light from the optical fiber at the location of the bend. In most cases, the influence of modal structure changes on bend loss is overlooked.
In single mode optical fibers general power loss is the primary consideration, because all leakage involves light in the fundamental mode of the optical fiber. However, in multimode optical fiber the modal structure affects the loss, with higher order modes suffering more loss than lower order modes. In addition, bends in the optical fiber cause modes to transform and mix. Accordingly, while a signal in a lower order mode may survive some bending, if it is converted to a higher order mode it will be more susceptible to bending loss.
The combination of higher order and lower order modes in a multimode optical fiber determines the bandwidth, and thus the signal carrying capacity, of the optical fiber. Bending multimode optical fiber may reduce the signal carrying capacity of the optical system.
The property of differential mode loss in multimode optical fibers can be more serious than generalized optical loss in single mode optical fibers. The latter can be addressed using low cost power amplifiers. However, differential mode loss in multimode optical fibers can lead to complete loss of signals propagating in higher order modes.